Soil & Tillage Research 107 (2010) 36–41

The impact of olive mill wastewater application on flow and transportproperties in soils

Mustafa Mahmoud a, Manon Janssen a, Nasser Haboub b, Abdallah Nassour c, Bernd Lennartz a,*a Institute for Land Use, Rostock University, Justus-von-Liebig-Weg 6, D-18059 Rostock, Germanyb Damascus University, Rural Engineering Department, Faculty of Agriculture, P.O. Box 10492, Damascus, Syriac Institute for Environmental Engineering, Rostock University, Justus-von-Liebig-Weg 6, D-18059 Rostock, Germany

A R T I C L E I N F O

Article history:

Received 14 August 2009

Received in revised form 27 January 2010

Accepted 27 January 2010

Keywords:

Olive mill wastewater

Water repellency

Infiltration rate

Hydraulic conductivity

A B S T R A C T

Olive mill wastewater (OMW) is the main waste product of the olive oil industry and is characterized by a

high salinity and high organic matter content. Until today, in several olive oil producing countries

untreated OMW is pumped onto agricultural land with possible adverse effects on soil properties. The

main objective of this study was to investigate the long-term effects of OMW application on soil

hydrophobicity, hydraulic conductivity and infiltration rate. In addition, the generation of transport

patterns was studied using dye tracer tests. The results showed that the regular application of OMW for 5

and 15 years increased soil hydrophobicity and decreased the drainable porosity (F < 30 mm), as a

consequence of a rising organic matter content. OMW application furthermore reduced the soil

hydraulic conductivity compared with a control site. Likewise, the infiltration rate decreased in the 5

years treatment compared with the control. The highest infiltration rate, however, was observed for the

15 years treatment because of the presence of large and deep shrinkage cracks that do not completely

close upon rewetting. The dye infiltration depth in the control was small and homogeneous compared to

the plots with OMW application, which fostered the generation of heterogeneous flux fields including

preferential flow conditions. As a consequence, fields irrigated with OMW are more prone to

groundwater contamination with substances either originating from the OMW itself or from agricultural

chemicals.

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1. Introduction

Olive (Olea europaea) oil production dominates the agriculturalsector in Syria. According to IOOC statistics, Syria is the world’sfifth olive oil producer with a production of 175 thousand tons in2004 (Al-Ashkar, 2007). The production of Syrian olive oil has beengrowing steadily in the last few years due to a continuousexpansion of cultivated area and the introduction of new high-yield olive tree species. An according increase in the quantity of by-products such as olive mill wastewater (OMW) is expected. Thewastewater arising from the milling process amounts to 0.5–1.5 m3 per 1000 kg of olives depending on the applied process(Paraskeva and Diamadopoulos, 2006). In total, 30 million m3

OMW are produced in the Mediterranean countries per year,accruing during a short time period after harvest (D’Annibale et al.,2004). The use of OMW – either treated or untreated – is thus animportant problem in the region. Untreated wastewater is often

* Corresponding author. Tel.: +49 381 4983180; fax: +49 381 4983122.

E-mail address: bernd.lennartz@uni-rostock.de (B. Lennartz).

0167-1987/$ – see front matter � 2010 Elsevier B.V. All rights reserved.

doi:10.1016/j.still.2010.01.002

used for irrigation purposes, but runs uncontrolled over the soil offields surrounding the mill in other cases.

OMW is characterized by a high salinity, high organic mattercontent, suspended solids and mineral elements as well as a highcontent of biomass growth inhibitors (Niaounakis and Halvadakis,2004). Several techniques have been proposed to amend OMW(e.g., Brunetti et al., 2007; Santi et al., 2008), but are not adoptedeverywhere. OMW application causes a change in the soil’sbiological (Saadi et al., 2007; Mechri et al., 2008), chemical andphysical properties (Jarboui et al., 2008; Lopez-Pineiro et al., 2008),and irrigation with wastewater in general has been shown todecrease the saturated hydraulic conductivity (Gharaibeh et al.,2007; Viviani and Iovino, 2004; Bhardwaj et al., 2008; Mandalet al., 2008), because of an ACCUMULATION of grease and oil inupper soil horizons (Micheal et al., 2008). Long-term application ofwastewater irrigation causes the accumulation of oil and grease inthe topsoil (Travis et al., 2008), which may enhance the soil’sdisposition to water repellency. Jarvis et al. (2008) found that soilwater repellency could enhance non-equilibrium water flow andsolute transport in structured clay soils. For a visualization of waterflow pathways, dye tracer experiments are a common method (e.g.,Flury and Fluhler, 1995; Mooney and Morris, 2004).

M. Mahmoud et al. / Soil & Tillage Research 107 (2010) 36–41 37

The main objective of this study was to investigate the long-term effects of OMW application on selected soil hydraulicproperties (hydrophobicity, saturated hydraulic conductivity andinfiltration rate) and on the generation of flow pathways.

2. Materials and methods

2.1. Experimental sites and sampling

The field sites are located in Saida village 15 km to the east ofDaraa in south-western Syria near the border with Jordan, at analtitude of 435 m a.s.l. The mean annual rainfall is 200 mm, theaverage temperature is around 35 8C in summer, and 12 8C inwinter (General Directorate of Meteorology in Syria, personalcommunication). Three sites were identified for experimentalPurposes: non-irrigated with OMW (T0) and regularly irrigatedwith untreated OMW for 5 and 15 years (T5 and T15). The oliveorchards at all sites were watered by drop irrigation during thegrowth period, but during the time period of olive mill operationfrom early October to late December the experimental plots T5 andT15 were irrigated by furrow irrigation with untreated OMW. Thevolume of OMW application varied from year to year depending onthe amount of olive oil production per year. The soil samples werecollected in December 2007/January 2008. Disturbed and undis-turbed soil samples were taken in 0–30, 30–60 and 60–90 cmdepth for each treatment at a water content lower than fieldcapacity. Disturbed samples were subjected to particle sizedistribution analysis (PSD). Undisturbed soil samples of 100 cm3

were collected for the determination of the soil water retentioncurve and saturated hydraulic conductivity (Ks). The field sites T5and T15 were plowed once or twice in spring and early summerand one to three times in late summer and autumn, while the T0site was plowed once in early summer and two times in latesummer and autumn. Frequency of plowing depends on theamount and number of OMW applications and on weed density.

2.2. Soil characteristics

The soils at all three sites are Cambisols according to FAOclassification (2006). They developed from weathered volcanicrock materials (Basalt and Pumice), which are characterized by ahigh content of calcium carbonate and accordingly exhibit amoderately alkaline pH (Table 1). Soil texture (determined withthe combined sieve and pipette method) is silt loam, and soil coloris 7.5 YR 4/3 throughout all investigated profiles. The bulk densityincreases with depth within each profile, and is lowest in thetopsoil of T5 and T15. Typically, the soils exhibit swelling andshrinking behaviour upon water status variation including thedevelopment of deep and wide cracks when dry. Naturally, the

Table 1Soil properties of the experimental sites with 0 (T0), 5 (T5) and 15 (T15) years of olive

Depth [cm] Clay [%] Silt [%] Sand [%] CO3 [%] pH Corg [g kg�1]

T0

0–30 24.2 74.5 1.3 16.38 7.93 2.3

30–60 21.9 76.5 1.6 17.00 7.90 1.4

60–90 17.1 79.5 3.4 18.44 7.84 1.1

T5

0–30 21.9 76.5 1.7 13.74 7.73 25.0

30–60 18.9 79.8 1.3 13.80 7.75 4.4

60–90 15.1 83.3 1.6 16.89 7.92 2.3

T15

0–30 18.6 78.8 2.6 18.33 7.81 39.1

30–60 20.1 77.6 2.3 21.71 7.88 5.3

60–90 18.2 79.9 1.9 25.38 7.91 5.2

soils are poor in organic matter. The typical clay mineral ismontmorillonite.

2.3. OMW characteristics

The OMW characteristics depend on the olive variety andripeness, climate and soil conditions and the oil extraction method.Selected physical and chemical characteristics of the OMW appliedat the experimental sites are given in Table 2.

2.4. Laboratory measurements

Total carbon and nitrogen were determined with the ElementarVario EL analyser (Elementar Analysensysteme GmbH, Hanau,Germany) according to DIN ISO 10 694 (1996) and DIN ISO 13 878(1998), respectively, while carbonate content was determinedseparately with the Scheibler equipment according to DIN ISO 10693 (1995; measurement of the volume of degassing CO2 afteraddition of hydrochloric acid).

The soil water repellency was determined by the water droppenetration time test (WDPT), which is a commonly acceptedtechnique to measure the degree of soil water repellency (e.g.,Buczko and Bens, 2006; Tarchitzky et al., 2007). Soil waterrepellency of all samples was measured at 21–23 8C and 50–60%relative humidity (Dekker, 1998). About 100 g of air-dried soil(<2 mm) was placed in Petri dishes and the surface wassmoothened manually. A total of 10–15 drops of distilled waterwas applied to the soil surface of each soil sample through amedicinal pipette (volume of water in one droplet: 58 � 5 ml), andthe actual time required for complete droplet infiltration wasrecorded. Six replications were conducted for each soil horizon.Two classes of water repellency were identified: (1) wettable, non-water-repellent soil (WDPT < 5 s) and (2) slightly water repellent soil(WDPT = 5–60 s) (Bisdom et al., 1993).

For the determination of the saturated hydraulic conductivity,Ks, 100 cm3 undisturbed soil core samples were used with sixreplications for each horizon. The samples were first wetted fromthe bottom to prevent air entrapment, and Ks was then determinedusing the constant head method (Reynolds et al., 2002).

Pressure plates were used to measure the water content ofundisturbed soil samples at field capacity (ufc) by measuringmoisture retention at�33 kPa. The bulk density was determined inall samples used for moisture retention measurements. Totalporosity (F) was calculated according to the following equation:

F ¼ 1� rb

rs

(1)

where rb is the soil bulk density [g cm�3] and rs the particledensity (2.65 g cm�3, the same for all treatments).

mill wastewater application. Bulk density: mean� standard deviation.

N [g kg�1] C/N Bulk density

[g cm�3]

Total

porosity (F) [%]

Soil drainable

porosity (Fd) [%]

0.8 2.86 1.26� 0.06 52.49 7.40

0.6 2.19 1.32� 0.02 50.05 7.23

0.5 2.14 1.34� 0.02 49.46 6.70

2.3 10.87 1.23� 0.04 53.52 6.47

0.9 5.09 1.26� 0.02 52.28 6.23

0.5 4.22 1.35� 0.02 49.07 4.83

3.9 10.03 1.22� 0.04 53.98 4.18

0.8 6.32 1.26� 0.01 52.47 3.60

0.7 7.82 1.31� 0.03 50.65 2.79

Table 2Selected physical and chemical properties of the olive mill

wastewater.

Parameters Values

pH 5.07

Electric conductivity [mS cm�1] 9.75

Dry matter [g l�1] 33.25

Organic matter [g l�1] 30.57

Mineral matter [g l�1] 2.69

Specific weight [g cm�3] 1.03

Soluble Cl� [mg l�1] 763.8

Soluble Br� [mg l�1] 18.42

Soluble NO3� [mg l�1] 1.19

Soluble PO42� [mg l�1] 426.12

Soluble SO42� [mg l�1] 174.48

Soluble Na+ [mg l�1] 128.8

Soluble K+ [mg l�1] 1050.9

Soluble Ca2+ [mg l�1] 137.5

Soluble Mg2+ [mg l�1] 168.3

Table 3Water drop penetration time (WDPT) of air-dried soil for the sites T0 (control), T5

and T15 (5 and 15 years of olive mill wastewater application, respectively), given as

mean� standard deviation.

Depth [cm] WDPT [s]

T0 T5 T15

0–30 <1af 25.2�4.3bg 36.1�7.8ch

30–60 <1df <1df <1df

60–90 <1ef <1ef <1ef

Superscripts indicate significant differences (P<0.05) between treatments. First

superscript represents differences between treatments; second superscript

represents differences between depths.

M. Mahmoud et al. / Soil & Tillage Research 107 (2010) 36–4138

The drainable porosity (Fd) is the sum of macropores andmesopores (diameter > 30 mm), it was calculated as the differencebetween the total porosity (F) and water content at field capacity(ufc) (Spychalski et al., 2007).

Fd ¼ F� u fc (2)

2.5. Field experiments

The infiltration rate at the soil surface was determined usingdouble-ring infiltrometers equipped with a Mariotte vessel(Reynolds et al., 2002). The inner and outer rings were 20 and40 cm in diameter, respectively, and the height was 25 cm. Fivereplications were conducted at each of the experimental sites. Inorder to obtain a proxy for the saturated hydraulic conductivity,Philip’s equation was fitted to the cumulative infiltration (Philip,1957):

I ¼ S �ffiffi

t2pþ C � t (3)

where I is the cumulative infiltration [cm], S the sorptivity [cm s�1/

2], t the time [s], and C a constant [cm s�1] approximating the field-saturated hydraulic conductivity.

Dye tracer experiments were conducted on two plots at eachexperimental site. An iron frame (0.8 m � 0.6 m � 0.17 m) wasused to delimit the infiltration area and was carefully pushed intothe soil to a depth of about 0.05 m. A Brilliant Blue FCF solution of5 g l�1 was applied. Brilliant Blue is nontoxic and thereforeparticularly suited for field use (Flury and Fluhler, 1995). Priorto dye application, the soil was moistured to field capacity. Onehundred millimeter of Brilliant Blue solution were applied. On thefollowing day, a soil pit was excavated and three vertical profileswere prepared at 20 cm intervals. Each section was photographedusing a digital camera (Sony Exilim 7; 2 Mega pixels) at a resolutionof 3072 � 2304 pixel. Subsequent image analysis followed theprocedure proposed by Janssen and Lennartz (2008). After ageometric correction, a band ratio was calculated by dividing thedigital number in the blue band by the one in the red band for eachpixel. The blue band depicts the stained soil pattern, while the redband represents the soil background; hence the band ratioincreases with dye concentration. For classification into stainedand unstained pixels, a threshold was determined in the band ratioimage. The result was a binary image, on which a median filter wasapplied and all objects smaller than 2 mm2 were removed. The dyecoverage distribution with depth was subsequently calculated byaveraging the three replications for each experiment.

2.6. Statistical analysis

The statistical analysis of data, especially the test on signifi-cance of differences between sample means (student’s t-test), fromboth laboratory and field experiments were performed using thesoftware package SPSS 15.

3. Results and discussion

3.1. Effect of long-term OMW application on soil chemical properties

As a consequence of long-term irrigation with OMW, theorganic C content in the topsoil increased from 2.3 g kg�1 in thecontrol to 25.0 and 39.1 g kg�1 after 5 and 15 years of OMWapplication, respectively (Table 1). The corresponding values fortotal N are 0.8, 2.3 and 3.9 g kg�1; the C/N ratio thus increased from2.9 to >10 after OMW application. An increase in organic mattercontent, total N and/or C/N ratio following irrigation with OMWhas been observed previously and may have a beneficial effect onsoil fertility and yield (Mekki et al., 2006; Brunetti et al., 2007;Sierra et al., 2007; Mechri et al., 2008). Furthermore, an increasingcarbonate depletion was observed in the topsoil compared withthe subsoil due to the low pH of the OMW (Table 2).

3.2. Water repellency

The results of the WDPT for the three experimental sites aregiven in Table 3. The differences between treatments T0, T5 andT15 were significant (P < 0.05) for the topsoil. The topsoil’spenetration time increased from <1 s in the control to 25.2 s (T5)and 36.1 s (T15) as a consequence of 5 and 15 years of OMWapplication. According to the classification of Bisdom et al. (1993),the topsoil of T5 and T15 exhibited a slight water repellency, whilethe soil was non-water-repellent in the topsoil of T0 and in thesubsurface layers of all treatments. The organic C content hadincreased at sites T5 and T15 (Table 1), and it was observed thatthe WDPT was related to the soil organic C content with a verysignificant correlation coefficient of R = 0.98. Rasa et al. (2007)reported that the water repellency was generally higher at the soilsurface, where organic matter had accumulated. The developmentof hydrophobicity with long-term wastewater application has alsobeen found by Wallach et al. (2005), and Gonzalez-Vila et al. (1995)reported that the hydrophobic behaviour of the topsoil enhances asa result of a long-term treatment with OMW. The increase in waterrepellency with long-term OMW application may be caused by thegeneration of hydrophobic components during the decompositionof organic matter; these components and residues of oil and greaseare wax-like substances that form a coating on soil particles(Bisdom et al., 1993). Tarchitzky et al. (2007) and Micheal et al.(2008) observed that soils irrigated with fresh water werehydrophilic, while those irrigated with wastewater exhibitedhydrophobicity. Water repellency may enhance preferential flowas well as surface runoff with subsequent erosion, thus entailing

Fig. 1. Boxplots of the saturated hydraulic conductivity Ks at different depths for the experimental sites T0 (control), T5 and T15 (5 and 15 years of olive mill wastewater

application).

Fig. 2. Boxplots of the final infiltration rates at the experimental sites T0 (control),

T5 and T15 (5 and 15 years of olive mill wastewater application).

M. Mahmoud et al. / Soil & Tillage Research 107 (2010) 36–41 39

important hydrological implications (Rasa et al., 2007). Soil erosionis a widespread problem in olive groves (e.g., Fleskens andStroosnijder, 2007; Barneveld et al., 2009; Gomez et al., 2009), andany factors facilitating its generation should thus be avoided.

3.3. Saturated hydraulic conductivity (Ks)

Fig. 1 shows boxplots of Ks at depths of 0–30, 30–60 and 60–90 cm for all treatments. A sharp decrease in hydraulic conductiv-ity could be observed at all depths with an extending period ofOMW application. The highest mean values were found in thetopsoil of T0 [0.23 cm h�1], while the lowest mean values wereobserved in the subsoil of T15 [0.04 cm h�1]. According to theStudent’s t-test, the results differ significantly (P < 0.05) betweenthe control (T0) and the treatments irrigated with OMW (T5, T15)at 0–30 and 30–60 cm depth, but not at 60–90 cm depth.Gharaibeh et al. (2007) reported that irrigation using wastewaterfor long periods reduced soil hydraulic conductivity as comparedwith non-irrigated areas.

The decrease of the saturated hydraulic conductivity is mostlikely the result of a reduction in the soil drainable porosity (Fd)(Table 1). Both soil properties are correlated with a significantcoefficient of R = 0.78. This reduction can be explained by twoprocesses: (i) suspended solids and organic matter could havepartially blocked soil pores (Bhardwaj et al., 2008) or swelling anddispersed clay particles can clog up small soil pores resulting inrestricted water movement in the soil (Tarchitzky et al., 1999), and(ii) the application of OMW resulted in a redistribution of soilporosity by a decrease of large pores and an increase of fine pores(Cox et al., 1997). In addition, high concentrations of K and Na inthe applied OMW (Table 2) probably led to an increase inexchangeable sodium percentage (ESP) and a subsequent degra-dation of soil structure, which in turn may have caused a decreasein hydraulic conductivity (Mekki et al., 2006; Jalali et al., 2008).Micheal et al. (2008) reported that oil and grease in wastewaterused for irrigation can accumulate in the soil and may lead to asignificant reduction in the soil’s ability to transmit water.

3.4. Infiltration rate

Fig. 2 shows boxplots of the field-equilibrium infiltration rate Cobtained by fitting Philip’s model for cumulative infiltration to themeasured infiltration data. The geometric mean was 1.36, 0.95 and2.75 cm h�1 for treatments T0, T5 and T15, respectively. The t-testconfirmed the significance of the differences between sites. The

decrease in infiltration rate after 5 years of OMW applicationcompared with the control can be explained by a reduction of thedrainable porosity, as has been described in the previous section.Furthermore, the high concentration of suspended solids andorganic matter in the OMW may have caused a sealing of the soilsurface and thus a reduction in infiltration rate (Abo-Ghobar, 1993;Viviani and Iovino, 2004).

Highest infiltration rates were observed in the 15-year-oldtreatment probably because of the presence of large and deepshrinkage cracks. A similar phenomenon was observed byGharaibeh et al. (2007), who reported that the infiltration afterwastewater application is altered by the presence of large and deepcracks that predominated and controlled infiltration with disre-gard to pore size distribution in the soil matrices. The higherorganic matter content in the treatment T15 may have hinderedshrinking cracks to completely close upon rewetting (Laegdsmandet al., 2005).

Fig. 3. Dye patterns at the sites T0 (control), T5 and T15 (5 and 15 years of olive mill wastewater application). The profiles are 45 cm deep and 76 cm wide. Two sections are

shown for each experimental plot.

Fig. 4. Average dye coverage for treatments T0 (control), T5 and T15 (5 and 15 years

of olive mill wastewater application).

M. Mahmoud et al. / Soil & Tillage Research 107 (2010) 36–4140

3.5. Dye tracer analysis

The dye tracer experiments were conducted in order to assessthe effect of changing hydraulic properties after OMW applicationon the generation of solute transport pathways. Black and whiteimages of the flow patterns at the three experimental sites arepresented in Fig. 3. The control site T0 showed a ratherhomogeneous dye distribution, indicating the dominance of matrixfluxes. The dye was concentrated in the topsoil and linearlydecreased from 95% coverage at the soil surface to <5% at 10 cmdepth (Fig. 4). The maximum penetration depth was 20 cm. In thetwo treatments irrigated with OMW, in contrast, the dye infiltrateddeeper, was more heterogeneously distributed, and distinctpreferential flow pathways could be observed. These pathwaysseem to follow cracks which establish along the polyedrical soilstructure. The average dye coverage at the soil surface of both T5and T15 was about 65% and hence distinctly smaller than in thecontrol; it decreased to about 15% at 10 cm depth (T5 and T15) and4% and 10% at 20 cm depth for T5 and T15, respectively. The dyeinfiltrated down to 40 cm depth at T5, and down to the bottom ofthe profiles (45 cm) at T15. The density of preferential flowpathways differed between both plots at T15, which was probablya consequence of varying size and distribution of the cracks. Yet,the general pattern was the same: While the topsoil was ratherhomogeneously stained, indicating dominant matrix fluxes, theshrinkage cracks formed preferential flow pathways and trans-ported dye down to the bottom of the profile.

The generation of preferential flow pathways at the sitesirrigated with OMW may have been caused by water repellency inthe topsoil (see Table 3), which delayed the soil-wetting process,prevented the development of a homogeneous infiltration front

and enhanced non-equilibrium flow. Taumer et al. (2005) found asignificant negative correlation between the water content andorganic matter content as well as water repellency. Mekki et al.(2006) have reported that the application of OMW decreased the

M. Mahmoud et al. / Soil & Tillage Research 107 (2010) 36–41 41

water content at field capacity. A lower water content is associatedwith a lower hydraulic conductivity of the soil matrix. As a resultflux heterogeneity upon ponding at the soil surface increasescausing the initiation of preferential flow. Furthermore, thereduction of saturated hydraulic conductivity in the topsoil (seeFig. 2) caused by blocking of pores and sealing of the soil surfacemay have reduced dye infiltration into the soil matrix.

4. Conclusions

The irrigation of olive groves with OMW for 5 and 15 years hasbeen shown to result in an increasing water repellency in thetopsoil due to the accumulation of organic matter and a decrease inhydraulic conductivity and infiltration rate. Water repellencydeteriorates the soil’s ability to retain water and thus reduces theplants’ water supply. In addition, all three factors may enhancesurface runoff and thus foster soil erosion. Dye tracer experimentsshowed that OMW application increased flux heterogeneity withthe generation of preferential flow pathways, increasing the risk ofgroundwater contamination. As a conclusion, long-term andintensive application of OMW should be avoided on slopedsurfaces and in areas with a shallow groundwater table.

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